Photographic process for etching figures on glass



Patented Sept. 12, 1944 PHOTOGRAPHIC PROCESS FOR ETCHING FIGURES ONGLASS John V. Sigford, Wabash, Ind., and Waldo H.

Kliever, Minneapolis, Minn.,

assignors to Minneapolis-Honeywell Regulator Company, Minneapolis,Minn., a corporation of Delaware No Drawing. Application August 29,1941, Serial No. 408,818

1 Claim.

Our invention relates broadly to an improved process for etchingwhereby, in the etching of figures in material in. general, thedifferential efiects are obtained from differential exposure to lightand the common methods of mechanically marking a resist are eliminated.

In particular, the invention relates to a new and novel photo-engravingprocess involving the etching of glass, quartz, vitreous enamel,plastics and other substances. A particular object of our invention isthe provision of a process for the photo-engraving and etching of glassto be employed as reticles for use in telescopes, range finders,microscopes, surveying and other optical or aligning instruments.However, it will be obvious that our invention is of broad applicationin the fields of photo-etching and of photo-engraving and accordingly isto be limited only by the scope of the appended claim.

In well known photo-engraving processes such as those embodying the useof copper or zinc plates for printing, the configuration to bereproduced by etching is photographed for the making of negatives fromwhich the etching resistpattern is produced on the metal. The metal iscoated witha light sensitive colloid, exposed under the negative to astrong actinic light and treated to produce the enamel etching resist ofa material such as insoluble glue or albumen. In the etching process themetal which is not protected is controllably dissolved by the action ofthe etching mordant while leaving the protected or enameled portions inrelief. Well known etching mordants employed in connection with thesemetals may consist of ferric chloride for the copper and nitric acid forthe zinc. In the usual methods for etching glass and optical material,an etching mordant such as hydrofluoric acid must be employed. It isfound that such a mordant will attack light sensitive coatings eventhough it hasbeen exposed and treated to form an enamel etching resistof the type successfully employed with metals. It is an object of thisinvention to provide a photo-etching and a photo-engraving process foretching in fine detail materials in general. A particular object of theinvention is the provision of a process for the photo-etching of glassand like materials that require the use of an etching mordant such ashydrofluoric acid which is harmful to resists of the type normallyemployed in connection with known photo-engraving processes.

A further object of the invention is to provide a process for thephoto-etching of materials which permits the use of an etching mordantsuch as hydrofluoric acid which will attack a light sensitive resist bythe use of a suitable resist in proper relation with the light sensitivematerial. For example, in accordance with our invention, the material tobe etched may be coated with silver to form a suitable resist forahydrofiuoric acid etching mordant. The well known photo-engravingprocesses referred to above may then be employed. Thus the portions ofbichromated colloid coating for the material when exposed to light undera suitable negative will become insoluble to form a suitable resist foran etching solution for silver or other first coating. The silver in theimage which is not protected by the enamel of the resist may accordinglybe dissolved away by immersion in a solution of sodium thiosulphate andpotassium ferricyanide known as Farmers reducer, or other mordant. Whenthe image is thus obtained on the material proper, it is rinsed, driedand may then be etched by the hydrofluoric acid mordant. The material isthen rinsed in concentrated nitric acid to remove any remaining enameland silver to thus complete the etching process.

Other objects and advantages will become apparent to those skilled inthe art and a more thorough understanding of our invention will beobtained by reference to the accompanying description of the applicationthereof to the production of reticles for use in telescopes and rangefinders.

In the practice of our invention as applied to the production ofreticles in glass, the glass material must be treated in accordance withknown processes to render-it chemically clean. The glass is thensilvered and this may be accomplished by the use of any standard processsuch as used in the making of mirrors including chemical deposition,cathodic sputtering, or by evaporation processes. The silvering may beaccomplished with very good results by the use of the well knownRochelle salts process, and in order to produce a perfectly resistantcoating it is preferable that at least three successive coating-s beemployed. It is found desirable to carry the silvering to a stage whereit has a slightly white appearance which occurs after the solution hasbecome sludgy. In this manner a surface is obtained to which the enamelwill better adhere than is the case with a perfectly smooth coating,although if the surface is permitted to become too white the enameldescribed below would be porous and non-resistant.

In the handling of telescopic reticles a sheet of plastic containingrecesses for holding the reticles in place may be employed. The cleaningand silvering may be accomplished by the use of trays such as used inphotographic work. In this manner they may be handled in quantities of,for example, two hundred at a time. cleaning .and silvering operationsmaybe performed in a period of about thirty minutes.

The silvered reticles are then coated with a light sensitive bichromatedcolloid illm which r.

iecting the image to the coating. A strong actinic light which may beproduced by an arc lamp may be employed with an exposure having aduration of about thirty seconds. In placing the image from thediapositive, or mask, with respect to thelight sensitive coating on theglass, we find it preferable to cut the diapositive to substantially thesame diameter as the reticle and with the image properly oriented toemploy a ring as a when exposed under the negative, forms the etch- 1frame to hold the negative and retlcle in posiing resist for thesilveretching process described below. The sensitizing solution mayconsist of sensitized resin which usually contains bichromateand isknown in general as Cold Top enamel.

tion. .A considerable number of reticles may be exposed-at a time. Theexposure must be of suiilcient duration and intensity to cause the.proper. reaction by the exposed light sensitive Other colloidalsubstances employed may be of ldcoating.

reticles and preferably made to flow evenly by a 20 centrifugal orspinning process. The reticle may be attached by a suction cup to adrill stock or motor which is rotated at a relatively low speed and thecoating material is applied to the center of the reticle. It isdesirable that the coating be as applied as evenly as possible and thethickness of the coat should be controlled because of its eifect on theexposure required. The thickness of the coatdepends on the temperatureand the speed of rotation. A light is employed that is not go and,rendered insoluble in the developer fluid,

sumciently bright for exposure purposes. The

' coating must be thin enough to give th desired detail and thick enoughto provide the required resists.

The coating thickness may be instantly gazed without stopping thespinner by observing the interference pattern on the surface. We havefound that the formation of a red interference disc of the first orderin the center of the rotating reticle, when dry, is indicative of athickthe surface while spinning and before the coating is applied islightly rubbed with a piece of cotton wet with alcohol. The coatingproass requires about three minutes and one person may convenientlyhandle about ten spinners at once.

The network and markings of the ilgure to be reproduced in glass asrequired in the formation of reticles for telescopes and range findersmay be drawn on paper at preferably many times actual size. In producingan accurate image in the negative it is necessary to so design thefigure contained in the drawing as to compensate fol lens imperfectionsand him halaticn by revision of the drawing in an amount determined-byex; perimentation. The figure may then be copied on microfilm or somehigh resolution film, for exjmple, by using a lens of short focal lcnsthpossessing excellent resolving power to provide the necessary sharp andline detail. To minimize tain sumcient density in the imase. Ourinvention is not limited to the use of photographic 7 The diapositive ofthe figure is then placed in P oper relation with the silvered and lightsensitive transparent coated glass for exposure to a light of highintensity for the purpose of in)? After-the light sensitive coating ofthe reticle has been exposed under the diapositive to light of strongactinic intensity the coated surface of the reticle is washed to developthe enamel resist. The developer may consist of alcohol particularlywhere a resin type colloid is employed, or of water where an albumen orglue colloid is used. Likewise.

a methylviolet dye. or other dye. may be mixed with the developer to dyethe cold top image so sion of about ten seconds and permitted to drain.As soon as the image is clearly seen the reticles are rinsed in waterand dried. The portion of the light sensitive coating that is exposed tothe light forms an enamel resist to protect the under coat of silver inthe subsequent silver" etching process. That portion of the coatingprotected from the lisht by the negative is dissolved awa leaving theimage contained in the unprotected silver coating. In this developmentprocess if the protective coating is weak and tends to come off in thedeveloper an underexpoe d condition is indicated. and if the mage doesnot raise an overexposed condition is indicated.

' The development is completed by washing in water andby lightlystroking the surface with a wad of cotton or a soft brush. A well knownan tendency of the enamel coa to blister.

The application of heat in drying may likewise serve to harden theenamel for betterwithstanding the action of theetching mordant.

The silver in the image which is unprotected by the enamel is etchedor.diseolved away by immersing the reticles in a solution of the 'wellknown Farmer's *reducer, containing sodium thiosulphate and potassiumferricyanide, al- M though other known agentsmay be employed.

This solution does not attack the Cold Top or other enamel resistand'this step of the process is terminated by complete transparency ofthe image on the exposed glass. Completion of I step may be accomplishedin about one minute following which the reticles are rinsed and dried.

The etching oi-the glass is preferably accomplished by-exposing thereticles to. hydrofluoric "acid fumes. This etching'mordant'will attackhalation, the exposure may be kept somewhat ,9." the Cold Top or otherlight sensitive enamel and lightv with copper intensification employedto n. elective resist is formed by the undercoab ing of silver. It isfound that a four oriilvosecond exposure of the reticles to fumes ofcom-5 inertial fifty-two percent acid. heated in awater bath't'oatemperature of 120 1-. will produce the dcsired'etching-of theglas'e. Tomaintain uniform strength of the acid fumes a lead container may beemployed, having a sliding cover on one'sideofwhichia-arecessfcrholdingthereticie,

and an the image.

that is may more readily be seen. The reticlcs are lifted from thedeveloper following an immerhardening solution may be employed toprevent.

The reticle is placed over the recess and the cover is moved over thecontainer for the required time, whereupon the reticle is removed andplaced in water to check the etching action.

A rinse of concentrated nitric acid, or other known solution, is appliedto the reticle for the removal of the silver and remaining enamel.

The etched glass is then washed, dried and filled in any well knownmanner, to enable the image to properly stand out, and thus complete theproduction of the reticle in accordance with the practice of ourinvention.

Our process may also be applied to etchin of .plastic materials, using asuitable organic solvent in place of the hydrofluoric acid, and someminor modifications in cleaning, etc., which will be obvious to thoseskilled in the art. For example, a cellulose acetate sheet might becoated 'with silver and light sensitive colloid and ex-- posed anddeveloped as described, leaving the silver film as a. resist, and thefigure etched in the cellulose acetate surface with acetone.

The application of. our invention to the proconfiguration of the ductionof reticles as disclosed above is intended to be merely illustrative.Other variations and applications of the invention will readily occur tothose skilled in the art. Therefore, our invention should be limitedonly in accordance with the scope of the appended claim.

We claim as our invention:

A process for Dhotographically applying an image to an article of glassor similar material, comprising, successively applying a plurality oflayers of silver coating to said article until the metallic surface hasa slightly white appearance, successively applying a plurality of layersof light sensitive coating to said silver coating until said coating hasassumed a thickness substantially equal to one-half the wave length ofred light, projecting an image oi! the desired configuration upon saidlight sensitive coating, and then developing said coating to remove thatportion of the light sensitive coating conforming to the image projectedthereon.

JOHN V. SIGFORD.

WALDO H. KLIEVER.

